The levels of mechanicalstrain most typically measured with strain gauges are very small and precise.Consequently, changes in resistance are also very small and thus cannot bemeasured directly with an ohmmeter. The strain gauge must therefore be includedin a measurement system where precise determination of the strain gauge'schange in resistance is possible. To do this, a Wheatstone bridge circuit mustbe created.

A strain gauge comprisesthe first component in this Wheatstone bridge circuit, as the strain gaugeconverts the mechanical strain into a change in electrical resistance. Both thestrain gauge and the measuring circuit are passive components. Each strain gaugeis then wired into a balanced bridge, consisting of two portions of an equalresistive value, formed into a Wheatstone bridge circuit. Regardless of bridgeconfiguration, energy must be passed through the gauge to excite the circuit.The circuit must have an input energy source to obtain a useful signal. Thisauxiliary energy is taken from a separate source. A constant electrical voltageis typically used, but a constant current power source can also be applied.

When even the slightestchange in strain gauge resistance due to a strain is detected, the bridgecircuit loses its symmetry and becomes unbalanced. A bridge output voltage is therebyobtained, which is proportional to the bridge's unbalance. If there were nochange in value to the balanced resistance, the electrical output would be zero.

On average, a strain gaugecan measure 1/10,000 micro strain, or enough to detect a small 1 dB vibrationacross a 10-ft room. Thus, measurement possibilities for various applicationshave, quite literally, an infinite range. An amplifier must be included in the measuringprocess to amplify the bridge output voltage to a level suitable for compatibilitywith indicating instruments or monitoring computers. Sometimes amplifiers aredesigned to give an output proportional to the bridge output in voltage.

Equipment Designs
The useof custom strain gauge technology in OEM medical devices and equipment involvesboth critical and non-critical applications, ranging from high-precisionrobotic surgery, to mammography machinery positioning, to patient scale weightdistribution and medical pump pressure and flow measurements. Some general examples of the hundreds ofsuccessful strain gauge technology applications developed by HBM are detailedbelow.

CAT Scan Machines. As a non-invasive medical device withcritical importance for accurate diagnostics within the field of radiology, CATscan machines require high-repeatability table positioning, as well as equalpatient weight distribution and precision movement of the CAT scan imaging device.Precision is required to perform high-accuracy imaging functions while preventingover-travel of the patient placed within the scanning tube. Within thisenvironment, the incorporation of a multi-axis strain gauge subassembly has proveneffective as a means of better ensuring the smooth, consistent movement andpositioning of the table, while adjusting for weight distribution. These designenhancements, created by the successful incorporation of HBM strain gaugetechnology, have facilitated the manufacture of more accurate medical diagnosticimaging machinery.

Mammography Machines. Mammography machines are among the medicalcommunity's most commonly used equipment for detecting breast tumors and otherabnormalities. For this type of application, an HBM medical equipment OEMrequired a means of monitoring the amount of physical force applied to thepatient by the machine itself when attempting to take an image. The proposedcustomer solution had to allow for the highest possible image resolution whilemaintaining patient position and comfort and preventing machinery over-travel.To solve this application challenge, the use of both dual and triaxial strain gaugeforce sensors with the incorporation of a redundant multi-axis sensor wasrecommended. The sensors were mounted on the top and bottom clamp of themammography machine, forming a flat item scale to monitor machinery flexure,while incorporating a mechanical stop to prevent overload protection andmeasurement redundancy. As a result, the OEM was able to introduce mammographymachinery design improvements which offered more accurate positioning, a higherdegree of patient comfort and enhanced image resolution.

Patient Lift Systems. Found predominantly within the European community,motorized lift systems are a common means of moving or transferring patientsfrom their beds into wheelchairs or gurneys. They are also use to turn patientsto minimize the potential for development of pneumonia or pressure ulcers. Thesystem consists of a handle device installed at a patient's bedside, which is pulledto activate motorized movement upon demand. By incorporating a custom strain gaugeforce sensing assembly within the lift system bed handle, a prominent medicaldevice OEM was able to achieve better control over system rate of movement,whereby a medical professional could proportionally apply force to the handleto achieve the most desirable motorized lift speed with greater ease of use.

Medical Weighing. Medical scale assemblies, such as thoserequired for pediatrics, veterinary medicine, home health monitoring andpharmaceutical use, are all examples of medical OEM applications which have successfullyincorporated the use of custom strain gauge technology. Highly variedrequirements of this type have ranged from subassemblies capable of measuring nanostrainor other values to a fraction of a gram, to weighing systems for measuring upto 500 lbf.

Remote Robotics Surgeries. Arecent general trend within the Asia-Pacific medical community has been theadoption of robotic methodologies for orthopedic surgery. In this type ofapplication, a physician is able to remotely operate on a patient whilemaintaining the same levels of precision and accuracy as an onsite surgicalprocedure. To keep pace with the demand for new robotic surgical equipment, amajor medical equipment OEM needed to be able to accurately measure the depthof force and drill bit rotational force when conducting remote hip surgeries. Themanufacturer needed to accurately assess how far into the bone to drill on anX-, Y-, and Z-axis, while maintaining the ability to move in and out of thedrill via the bit with highly repeatable and accurate robotic positioning. Positioningin this context refers to the "in/out" twisting motion of the drill, indicativeof a rotational torque measurement, with manufactured accuracy requirements totens of thousandths of an inch. To address these application requirements, aseries of multi-axis custom strain gauge sensor subassemblies were designed inboth compression and tension modes to measure downward and upward force andmotion, while another strain gauge sensor was mounted in a perpendicularconfiguration to measure full deflection, drilling motion consistency andensure patient positioning on the operating table. As a result, the medicalequipment OEM was able to introduce a high-precision robotic surgical devicewith greater accuracy and performance capabilities.

Medical Pumps. Custom strain gaugetechnology has been widely incorporated into medical infusion devices, such asinsulin pumps and dialysis machines, as a means of more accurately predictingfluid flow and ensuring a constant stream of vital medication to the patient. Forfluid flow monitoring, a medical device OEM required a 1.5 lbf strain gaugesensor assembly which was positioned and strategically weakened to form ablade-shaped configuration. With its blade shape, the assembly mimics thebehavior of a perfect spring, returning to zero when liquid stops flowing. Useof this specific technology within finished medical devices has been adopted bynumerous medical equipment OEMs as a cost-effective means of predictingcritical liquid flow.

Robert Chevalier is director ofsensor sales for HBM Inc. Molly Chamberlin, president and founder of EmbassyGlobal PR and Marketing Communications, LLC, co-wrote this article.

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